Mechanical and cryogenic spiral freezers are well known in the food industry. The spiral constructions of these freezers permits relatively long freezing surfaces to be contained in a relatively small area. Such freezers, however, include a straight entry section which undergoes a transition to a circular configuration within the freezer. At the transition point the conveyor belt usually undergoes two different changes at the same time. The inner section of the belt compresses (collapse's) while the outer part expands and the links pull apart. U.S. Pat. No. 5,141,099 describes an expand only belt. These conditions squeeze or pinch the bottom of the food item that is on the inside of the conveyor and also makes it stick in the opening of the conveyor belt which damages the food item and makes it difficult to release. In contrast, the outside conveyor expands and stretches or pulls the food item out of a round shape. Both conditions create an undesirable frozen shape which is difficult to stack and package. Retaining the round configuration and flat bottom of a meat patty is of prime importance to attain proper packaging as well as grill to be able to "sear" the surface during cooking.
Hybrid freezing systems have been proposed, such as that set forth in U.S. Pat. No. 5,220,803 to Kiczek. The hybrid freezing system of the Kiczek patent mates the outlet of an inversion-type cryogenic freezer to the inlet of a spiral-type mechanical freezer. Cryogen vaporizing in the immersion unit is directly injected into the mechanical refrigerator where the cryogen is in direct heat exchange with refrigerated air circulating inside the mechanical refrigerator.
As shown in FIG. 1 of the Kiczek patent, the product to be frozen is dropped through a port into the liquid cryogen and is then carried by a conveyor belt through the cryogen bath to the entrance of the mechanical freezer. During passage of the food product through the cryogen bath, a crust is formed on the product which minimizes or lowers the dehydration of the final frozen product. It would not be practical to utilize this method for fragile food products such as hamburgers, which need to be placed in a certain and fixed relationship to each other on the conveyor belt.
Accordingly, it is a principle object of the present invention to provide a freezing system to impart a frozen crust on fragile food products, such as hamburgers, prior to introducing the food product into a mechanical or cryogenic spiral freezer.